The endurance capacity of muscle varies with the task that is performed. We found that the endurance time for a submaximal isometric contraction with the elbow flexor muscles was twice as long when the wrist was attached to a force transducer compared with when it supported an equivalent inertial load. Although the subject sustained a constant force when the wrist was restrained by a force transducer and maintained a constant elbow angle when supporting the inertial load, the resultant muscle torque and the rate of increase in the average EMG were identical for the two tasks. Nonetheless, additional results suggested that the descending drive to the motor neurons was greater during the constant-position contraction. We hypothesize that endurance time of the elbow flexor muscles is less for a constant- position contraction compared with a constant-force contraction due to greater excitatory descending drive to the motor neurons and greater inhibitory feedback from the muscles. According to this hypothesis, the difference in endurance time for the two tasks is attributable to differences in the input received by the spinal motor neurons. We propose three specific aims (Aims 1 to 3) to examine the, descending- drive component of the hypothesis and two aims (Aims 4 and 5) to assess the inhibitory-feedback component. The hypothesis predicts that motor unit activity will be greater during the constant-position contraction (Aim 1) and that endurance time will be briefer when the gain of the position-feedback signal is increased (Aim 2) and vibration is applied to the active muscles (Aim 3). Furthermore, the hypothesis predicts that the decline in maximum discharge rate of motor units in the contralateral muscles (Aim 4) and that the increase in mean arterial pressure (Aim 5) will be greater after the constant-position contraction. We are not aware of another study that has examined the contribution of neural mechanisms to the fatigue experienced during constant-force and constant-position isometric contractions. The outcomes will provide novel information on the physiological adjustments that occur during isometric contractions, which are the most common form of muscle activity, and will have direct application to the design of work tasks in ergonomics and the prescription of physical activities in rehabilitation.
